Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble in such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating. Thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution, plasma gases, or have metal or metal composites deposited in the spaces of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a patterned substrate surface. In some instances, it is desirable to heat treat the remaining photoresist layer, after the development step and before the etching step, to increase its adhesion to the underlying substrate.
In the manufacture of patterned structures, such as wafer level packaging, electrochemical deposition of electrical interconnects has been used as the density of the interconnects increases. For example, see Solomon, Electrochemically Deposited Solder Bumps for Wafer-Level Packaging, Packaging/Assembly, Solid State Technology. Gold bumps, copper posts and copper wires for redistribution in wafer level packaging require a resist mold that is later electroplated to form the final metal structures in advanced interconnect technologies. The resist layers are very thick compared to the photoresists used in the IC manufacturing of critical layers. Both feature size and resist thickness are typically in the range of 2 μm to 100 μm, so that high aspect ratios (resist thickness to line size) have to be patterned in the photoresist.
Devices manufactured for use as microelectromechanical machines also use very thick photoresist films to define the components of the machine.
Positive-acting photoresists comprising novolak resins and quinone-diazide compounds as photoactive compounds are well known in the art. Novolak resins are typically produced by condensing formaldehyde and one or more multi-substituted phenols, in the presence of an acid catalyst, such as oxalic acid. Photoactive compounds are generally obtained by reacting multihydroxyphenolic compounds with naphthoquinone diazide acids or their derivatives. Novolaks may also be reacted with quinone diazides and combined with a polymer. It has been found that photoresists based on only novolak/diazide do not have the photosensitivity or the steepness of sidewalls necessary for certain type of processes, especially for very thick films.
It is the object of the present invention to provide a chemically amplified photoresist useful for imaging films as thick as 200 microns, which provides good lithographic properties, particularly photosensitivity, high aspect ratio, vertical sidewalls, improved adhesion on metal and silicon substrates, compatibility with electroplating solutions and process, reduced resist film cracking and improved environmental stability. The inventors of the present invention have found that a photoresist comprising a polymer which is insoluble in an aqueous alkali developer but becomes soluble prior to development, a photoacid generator which produces a strong acid upon irradiation and a photobleachable dye which is absorbing at the same radiation wavelength(s) as the photoacid generator and preferably, has a similar or lower rate of photobleaching compared to the photoacid generator, provides the desired lithographic properties when imaged, especially for thick films up to 200 microns.